# Waves Physics AS

waves physics AS

## Wave definations

progressive wave - a wave that transfers energy from one position to another.

Stationary wave - a wave that does not transfer energy from one position to another.

Displacement - the distance of a point on a wave from the waves starting position.

Amplitude - maximum displacement of a wave.

Wavelength - the distance from one point on the wave to the next exactly similar on the wave.

Period - the time taken for a complete oscillation of the wave.

Frequency - the number of complete oscillations per second.

## More definations

Mechanical waves - waves that have to travel through a medium e.g. sound in air.

Tranverse waves - waves in which the particles vibrate perpendicular to the direction of travel of the wave. e.g. electromagnetic waves

Longintudinal waves - waves in which the particles vibrate paralell to the direction of travel of the wave. e.g. sound.

## Phase and phase difference

Although all particles in the wave vibrate in the same pattern they do not move in step with each other, they are out of phase.

The phase difference is the amount one point in the wave leads or lags behind another. It can be measured in degrees or radians.

A ponit exactly one wavelength out of phase with another point is in phase.

A point exactly half of one wavelength is exactly out of phase with the other point.

You can calculate phase diffrence by ((position of point 1 x position of point 2) / wavelength) x360.

## Wave energy, intensity, amplitude and wave speed.

Energy is transferred by neighbouring particles vibrating and passing along the energy, the particles themselves do not move.

Intensity is the amount of enrgy transferred per unit area perpendicular to the wave velocity. It is calculate by Intensity = power / cross -sectional area.

Intensity is proportional to amplitude squared. So when the amplitude doubles the intensity quadruples.

Velocity = distance / time = wavelength / time

time = 1/ frequency

So velocity = wavelength x frequency.

## Uses of electromagnetic radiation

Radio - broadcasting, astronomy and MRI

Microwave - cooking , telecommunications,radar.

Infrared - remote controlling, night vision,cooking.

Visible - photography,signalling.

Ultra violet - steriliation , marking possesions,suntanning.

X - rays and gamma rays - medical imaging and treatment, sterilisation

## Ultra Violet

Ultra violet light damages skin cells and causes cancer.

There are three types.

UV-A - 400 - 320nm- this ages the skin and damages eyes. 99% of UV light at sea level is UV-A.

UV-B - 320 - 280 nm - this damages DNA in the skin cells and cause cancer, it damages eyes. Mostly absorbed by the ozone layer. This is needed by the body to help produce Vitamin D.

UV-C - 280nm - this is harmful to the eyes.

## Polarisation

Electromagnetic waves are periodic variations in the electric and magnetic fields in space. The 2 fields vary at 90 to each other and the direction of wave travel.

Unpolarised waves vibrate in all direction. You can pass the waves through polarising filters which are long chain polymers. These absorb energy from the oscillating electric field. So if the polymers were arranged vertically they would absorb the vertical vibrations of the wave, only the horizontal vibrations would pass through and the wave would be planar polarised. If you have two polarising filters at right angles to each other no wave will pass through.

These are used in photography, sunglasses and to find stresses in a material. Due to reflection and scattering most sunlight at sea level is polarised.

Malus's law - A = A0 cos theta and I =I0 cos^2 theta

## Superposition

This is where two or more waves meet at a point and combine together.

Priciple of superposition-

When two or more waves meet at a point their resultant displacement will be the algebraic sum of their original displacements.

E.g. if the orginal displacement of wave was 1 and the other -1 the resultant displacement would be 0.

or if the original displacement of a wave was 1 and the other also 1 the resultant displacement would be 2.

## Diffraction

This is where a wave goes through a gap and spreads out. The amount it spreads out depends on the relationship between the wavelength and the width of the gap. Maximun diffraction - the wavelength = the width of the gap.

Diffracting sound- the wavelength of sound is from a few mm to a few m so a doorway will diffract sound so the noise will spread in the next room.

Diffracting light - The wavelength of light is too small to be diffracted by a doorway. You can diffract light using a laser and a slit.

DIffracting Radio waves- the wavelength is about a km.Top of building or hills perfect, in a car the car window is too small to diffract so an arial is used.

Diffracting microwaves - the wavelength is around an dcm so not easily diffracted, they mostly travel through space in straight lines.

## Interference

Can be used as proof of wave behavior.

Constructive interference - 2 waves at same point with same amplitude makes double big wave (superposition)

Destructive Interference - when waves cancel each other out.

These depend on phase difference and so path difference. This is the extra ditance travelled by one of the waves compared with the other. This determines whether at a point the distance is a multiple of the wavelength or not and so phase difference.

## Coherence

2 coherent sources have constant phase difference. This is only possible if they have the same frequency.E.g. from a laser using 2 slits.

Young's double slit - This is usd to measure the wavelength of light.

Wavelength = (slit separation x fringe separation) / disstance to screen from slit

The fringe pattern occurs when the light rays meet the screen with diffrent path differences. Where there is a constructive interference there is a bright spot and where there is destructive interfernce there is a dark spot.

Diffraction gratings - many slits much interfernce but still a fringe pattern. To find wavelength use distance between slits x sin(theta) = order of maxima x wavelength

## Stationary waves

Nodes - points on the stationary wave that do not move.

Antinodes - points on the stationary wave with maximum displacement.

**A standing (stationary) wave is produced when two progressive waves of equal wavelength and amplitude, travelling in opposite directions superimpose.**

In a stationary wave sections are in phase with themselves but not other sections.The distance between adjacent nodes is half the wavelength.A stationary wave only works when the tube (or string) is a multiple of a quarter of the wavelength. Because of systematic error in a tube wavelength = 2(2nd length - 1st length)

The fundamental frequency (mode of harmonic) - is the lowest frequency in a harmonic series where a standing wave forms.

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